OFFER THREADS IMPLEMENTATION:
1) Added Random Chunk scheduler in get_chunks_to_accept (LATEST scheduler
should work, but I have never tested it).
2) Removed check on lossy path when sending an ACK (I want the ACK to be
received to start a new offer thread).
3) Check active transactions list and purge expired ones. This is done
whenever a chunk is received.
4) Modified send_offer() to send offers to neighbors in a Round Robin
Signed-off-by: Stefano Traverso <email@example.com>